just tried and cp -a <source> <dest> and it does work as expected. Are you getting an error? if you're trying to copy the whole disk you will more then likely need to run as sudo. also check man cp for more information.
– hossAug 6 '13 at 18:20

Download Carbon Copy Cloner 2.3. It is compatible with your version of MacOS X and processor. This solid software will preserve all the ACL, all the permissions and all the hidden files of your whole file system.

Once you have made your external copy, check it with the originalDisk Utility.

Don't use a more recent version of Disk Utility which will want to fix many discrepencies…

Another useful method is to use cpio. It can preserve hard links and can handle some long filenames which are not supported into tar.
You can check more details here for a nice comparison between the three techniques.

I believe all of the methods word, you should just fine that suits your needs and do the job more efficiently.